1. Field of the Invention
The present invention relates to a power conversion system, and more particularly to a power conversion system for controlling harmonics, which can decrease the current harmonics distortion rate.
2. Description of the Related Art
Most of the regenerated energy source outputs the DC power, which is converted into the AC power through the DC-AC power converter with parallel configuration. Thereby, the output electric quality is very important for the electric power system, and it is a few ideas and plans for the related technology about the power converter.
Taiwan Patent Application Number 100106112 discloses a transformer with new type of the refraction line, and it can eliminate the odd harmonics expect for 24n±1 orders. Thereby, the transformer with the new type of the refraction line is the passive element and adds the extra size and cost. Moreover, the majority application range is the voltage source inverter and its application is limited; Taiwan application number 99136014 uses the 3-phase multilevel current source transform with the parallel configuration to overcome the specific harmonics, and however, there is the circulation problem in the 3-phase multilevel current source transform with the parallel configuration. Thereby, the zero order sequence current harmonics generated is solved by the extra compensation control strategy of the zero order sequence harmonics; Taiwan application number 99114137 changes the PWM signal to eliminate the low order harmonics, and however, it must consider that the load character of the low pass filter in practice. Generally speaking, the higher load feature impedance needs to match the element with higher inductance, and therefore, it must to know the load character to design the low pass filter so as to effectively eliminate the harmonics. Thereby, the method can not be used widely; Taiwan application number 95113130 uses the PWM and the random sampling to eliminate the harmonics, and however, the method uses the Fourier analysis and the random sampling theory to eliminate the switched angles of the different harmonics. Thereby, it makes the load heavy and is unfit for the controller because there are too many nonlinear equations and the switched angle solutions are obtained by the iteration; Taiwan Patent Application Number 99143779 uses the vector control unit to eliminate the zero phase sequence harmonic wave, and however, it needs the extra voltage detection device including the adder and the controller when the vector control unit receives the 3-phase voltage status. Thereby, it is easy to generate the operation uncertainty because the extra hardware such as an adder and a controller are added.
European Patent Number 0319910B1 discloses harmonics suppression control circuit for a PWM inverter, and it uses the voltage at the feedback load terminal to analyze the harmonics element. Thereby, it can control PWM signal by checking the table so as to decrease the specific harmonics element. However, because there are a wide variety of loads to choose from, the information of the switch angle of PWM is too large to decrease the assigned harmonics element compensation stored in memory. It will bring the confusion for the digital signal processor with the limited memory capacity. European Patent Number 0906654B1 discloses power line harmonics reduction by hybrid parallel active/passive filter system with square wave inverter and dc bus control, and it uses the mixed filter with parallel configuration to decrease the harmonics element of the system. However, because the passive filter is composed of the capacitors and inductors, the size of the filter must be large if it performs to filter the harmonics current with lower frequency. The current controller of the synchronously rotating coordinate axis is built by the hardware as the active power filter, and therefore, the electric elements are easily affected by the age and the environment, for example, temperature, humidity, etc., and it is easy to damage the controller or cause the bad performance of the controller. U.S. Patent Publication Number 20130076293 discloses Mid-voltage variable-frequency driving system and total harmonics distortion compensation control method, and it adjusts the current reactive power of AC side in the 3-phase power factor rectifier to decrease the current harmonics element in the AC power. Because the distortion compensation method of the total current harmonics generates the total current harmonics distortion compensation conditions by feeding the current of the AC power back, analyzing and calculating the total current harmonics distortion rate, the angle difference between the voltage and current of the AC power and the power factor of the AC power so as to adjust the reference instruction value of the reactive power, and therefore it needs to analysis the three conditions to adjust the reference instruction of the reactive power for compensating the total harmonics distortion so as to increasing the calculation load of analyzing the program.
For the reason that the conventional method and device could not effectively solve the mentioned problems, a need has arisen to propose a power conversion system for controlling harmonics which effectively eliminate the harmonics.